refactor(server): use Latch for shutdown gate

[kitlangton]

What

The server process's shutdown signal is a pure "shutdown requested" gate — a Deferred<void> that is only ever succeeded and awaited. Latch states that intent directly: a valueless, never-failing open-once signal.

How

packages/server/src/process.ts:

• shutdown is now yield* Latch.make() (starts closed).
• The lifecycle callback receives shutdown.open.pipe(Effect.asVoid) instead of Deferred.succeed(...).pipe(Effect.asVoid).
• Both waiters (shutdown handle returned to callers, and the raceFirst against boot) use shutdown.await.

No behavior change. Found by a repo-wide audit of Deferred<void>-as-gate patterns; sibling PRs convert the other pure-gate sites.

Testing

• packages/server: bun typecheck, full bun run test (22 pass / 0 fail).